linux-kernel - Re: [PATCH v3 5/7] sched: SIS_CORE to disable idle core search

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Date:   Fri, 28 Jun 2019 15:29:46 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Parth Shah <parth@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...hat.com, tglx@...utronix.de,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, tim.c.chen@...ux.intel.com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH v3 5/7] sched: SIS_CORE to disable idle core search


On 6/28/19 12:01 PM, Parth Shah wrote:
>
> On 6/27/19 6:59 AM, subhra mazumdar wrote:
>> Use SIS_CORE to disable idle core search. For some workloads
>> select_idle_core becomes a scalability bottleneck, removing it improves
>> throughput. Also there are workloads where disabling it can hurt latency,
>> so need to have an option.
>>
>> Signed-off-by: subhra mazumdar <subhra.mazumdar@...cle.com>
>> ---
>>   kernel/sched/fair.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index c1ca88e..6a74808 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6280,9 +6280,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>   	if (!sd)
>>   		return target;
>>
>> -	i = select_idle_core(p, sd, target);
>> -	if ((unsigned)i < nr_cpumask_bits)
>> -		return i;
>> +	if (sched_feat(SIS_CORE)) {
>> +		i = select_idle_core(p, sd, target);
>> +		if ((unsigned)i < nr_cpumask_bits)
>> +			return i;
>> +	}
> This can have significant performance loss if disabled. The select_idle_core spreads
> workloads quickly across the cores, hence disabling this leaves much of the work to
> be offloaded to load balancer to move task across the cores. Latency sensitive
> and long running multi-threaded workload should see the regression under this conditions.
Yes in case of SPARC SMT8 I did notice that (see cover letter). That's why
it is a feature that is ON by default, but can be turned OFF for specific
workloads on x86 SMT2 that can benefit from it.
> Also, systems like POWER9 has sd_llc as a pair of core only. So it
> won't benefit from the limits and hence also hiding your code in select_idle_cpu
> behind static keys will be much preferred.
If it doesn't hurt then I don't see the point.

Thanks,
Subhra